1. Field of the Invention
The present invention relates to a semiconductor memory device, and in particular, relates to a semiconductor memory device which performs self-refreshment.
Priority is claimed on Japanese Patent Application No. 2007-331149, filed Dec. 21, 2007, the contents of which are incorporated herein by reference.
2. Description of Related Art
In recent years, a semiconductor memory device (DRAM) has been used in a portable data terminal. In a portable data terminal, the power voltage is supplied from a battery. Therefore, in order to use the terminal continuously for a long time, it is necessary to reduce the power consumption as much as possible (see for example, Japanese Unexamined Patent Application, First Publication No. H09-7367).
That is, a semiconductor memory device as shown in FIG. 5 has been put to practical use, and not only has an auto-refreshment function for refreshing memory cells of the entire memory area in the semiconductor memory device, but also has a partial self-refreshment (called “self-refreshment” below) function in which a target memory area (e.g., a bank) for refreshment can be determined so as to further reduce the power consumption in consideration of the use of the semiconductor memory device in a portable data terminal.
Therefore, the semiconductor memory device has two refreshment modes: auto-refreshment and self-refreshment.
In the conventional semiconductor memory device, a command decoder 2 interprets a command from an external device, and writes type data, which indicates the refreshment mode, in a mode register 3, so as to enter the refreshment mode.
The present inventor has recognized that when the type data is written and set in the mode register 3, there is a delay from the writing of the type data to the output of the set type data.
When the command decoder 2 detects the type data of refreshment by means of a refreshment command input from an external device, the command decoder 2 performs the setting of the type data to the mode register 3 and the output of a refreshment signal MREF to a control circuit 4 synchronously.
When the output timing of the type data from the mode register 3 to the control circuit 4 is delayed from the input timing of the refreshment signal MREF into the control circuit 4 by a delay time from the input to the output of the type data at the mode register 3, then the refreshment signal MREF is input into the control circuit 4 before the setting of the mode register 3 is communicated to the control circuit 4.
Therefore, when the refreshment mode is entered, type data which indicates the auto-refreshment is set in the control circuit 4, and the refreshment mode of the auto-refreshment is effective until the auto-refreshment is completed and the next command for performing the self-refreshment is entered.
Therefore, the refreshment mode is switched from the auto-refreshment to the self-refreshment after all memory cells of the semiconductor memory device are subjected to the auto-refreshment at least once, that is, after at least one cycle of the relevant refreshment process is completed.
In the above-described structure, even when a refreshment command for switching the refreshment mode from auto-refreshment to self-refreshment is input during the auto-refreshment, the control circuit 4 cannot switch the refreshment mode to the self-refreshment until the refreshment of all memory cells of the semiconductor memory device is completed.
Therefore, the present inventor has recognized that in the conventional semiconductor memory device, even when a refreshment command for switching the refreshment mode from auto-refreshment to self-refreshment is input during the auto-refreshment:    (i) target memory cells for self-refreshment are not refreshed for a long period of time (especially, in a synchronous-type semiconductor memory device having a direct auto-refreshment function); and    (ii) the refreshment mode is not switched to self-refreshment until the refreshment of all memory cells are completed, and thus no current-reduction effect for a target memory area for refreshment is obtained immediately after the self-refreshment mode is entered.